Why is my flat rooflight leaking?

Those two culprits account for the large majority of wet flat rooflights, and they call for completely different responses. Before you assume the worst, it is worth working out which one you are actually looking at, because the cure for one does nothing for the other.

The timing gives it away. Condensation appears in cold weather, usually in the morning, and often after a steamy session in a kitchen or bathroom below. It gathers on the coldest surface it can find, which on a rooflight is the frame or the cold edge of the glass, then beads up and falls. It has nothing to do with whether it is raining.

A real leak follows the weather instead. It shows during or just after rain, turns worse in wind-driven rain coming from one direction, and tends to leave a spreading brown tide mark rather than clean water. If you can catch it in the act, look at where the water starts. Condensation weeps evenly from the inner frame and glass edge. Rain ingress usually tracks from one corner or one side and runs down the plasterboard reveal.

Condensation is a room problem more than a rooflight problem. Warm moist air meets a cold surface and gives up its water. An older aluminium frame with no thermal break is a cold bridge that practically farms the stuff. The answers are better background ventilation, an extractor that actually runs while you cook or shower, trickle vents left open, and where the unit itself is the cold spot, a replacement with a warm-edge, thermally broken double-glazed unit that keeps the inner face closer to room temperature. Where a hot south-facing room is driving the humidity as well as the temperature, the same thinking behind our energy-efficient skylight installation work applies: the glass specification and the frame both change how the room behaves.

A flat rooflight does not sit flat on the deck. It sits on a raised kerb called an upstand, built from timber or a proprietary insulated frame, and the roof covering is dressed up the sides of that kerb. The unit then clamps or seals onto the top. This junction is where most genuine leaks live.

A few things go wrong here. The upstand may be too low, so water lying on the roof can reach the seal and work its way over the top. Good practice puts the upstand at least 150mm above the finished roof surface for exactly this reason. The gasket or seal between the unit and the kerb can perish, hardened and cracked by years of ultraviolet and heat. And the whole assembly moves: on a south-facing flat roof the sun heats the unit through the day and lets it cool at night, so it expands and contracts daily, and that constant working loosens a seal that was sound when it went in. If the upstand is too shallow or the unit that sits on it has reached the end of its life, raising the kerb and fitting a new unit onto it is a replacement, and that is our work.

A flat roof is never truly flat. It is laid with a slight fall so water runs to an outlet. When that fall is wrong, or an outlet is blocked with leaves and grit, water ponds around the upstand and sits there for days. Standing water finds any weakness a running film would pass straight over, and it degrades the covering faster than anything else.

So look at the covering around the rooflight. Felt, EPDM rubber, GRP fibreglass and single-ply membranes all fail in recognisable ways: splits along a stress line, blisters where moisture is trapped underneath, and lifted laps or lifted flashing where the covering turns up the side of the upstand. Any of those close to the kerb will feed water straight to the timber and into the room below.

Here is the honest dividing line. If the leak is coming from the flat roof covering itself, that is roofing work, and a roofer is the right call to put the covering right. If the rooflight is coming out anyway, that moment, with the roof open, is the sensible time to sort the whole detail together rather than in two visits. We set out how the unit and its upstand are dealt with on our flat roof skylight installation page.

Sometimes the water is the unit telling you it is spent. Misting or a milky haze between the two panes of glass is not a leak from outside at all. It is a failed edge seal on the sealed unit, letting the dry gas out and damp air in, and once that has gone the unit cannot recover. Replacing that failed unit is a replacement, and it is our work.

Two more mechanisms sit inside the frame. Many rooflight frames carry small drainage channels, sometimes called weep holes, that are meant to carry away the trickle of water that gets past the outer seal. Block those with dirt or moss and the water backs up until it spills over the inner edge, which looks for all the world like the glass is leaking. And a perished frame gasket lets wind-driven rain past the first line of defence entirely. Neither of those is a job for sealant smeared over the top. A tired unit on a flat roof has usually reached the point where the whole rooflight is better replaced.

Start with the diagnosis, because it decides everything. Watch when the water arrives. Cold still morning after a hot shower points at condensation, and better ventilation with a warmer unit is the route. Water during rain, worse in wind, points at ingress, and then you check the upstand height, the seal onto it, and the covering around it in that order.

If the flat roof covering has split or the falls are ponding, call a roofer for the covering. If the rooflight itself has failed, a blown sealed unit, a perished seal, or an old unit sitting on too shallow an upstand, replacing it is what we do, and we deal with the upstand at the same time so the new unit sits on a sound detail rather than the one that let you down. Tell us the roof size, the rough opening and which way it faces, ring 01702 898232 or request a quote, and we will look at the whole roof, not just the glass.